California’s Carr Fire spawned a true fire tornado | Science News for Students

California’s Carr Fire spawned a true fire tornado

Not a common fire-whirl, these are amazingly rare freaks of nature
Nov 14, 2018 — 1:53 pm EST
a fire truck heading towards the Carr Fire

Dramatic image of a fire truck going in to fight the Carr Fire outside Redding, Calif., in late July. On July 23, the intense heat and debris from this fire created a fire tornado.

E.L. Karey

What’s scarier than a tornado? How about a tornado made of fire? On July 26, 2018, the so-called Carr Fire outside Redding, Calif., spawned the strongest tornado in state history: a fire tornado, or firenado.

This rare and terrifying phenomenon was only the second true fire tornado in recorded history — and the first witnessed in the United States.

Wildfires have become an all too common phenomenon in California. The region’s low humidity and scarce rainfall make it an environment ripe for blazes. In fact, much of the state should burn naturally every 50 to 100 years or so. An occasional fire can even help the ecosystem. It’s a way for nature to restore nutrients to the soil while cleansing the landscape of an overgrowth of moisture-robbing vegetation. But people have been building homes in these regions. So when a forest goes up in flames, so too can houses. (Witness the estimated 6,000-plus homes destroyed by the so-called Camp Fire, this month, in Paradise, Calif.)

The Carr Fire was first reported on July 23, west of Redding, Calif. An RV trailer suffered a flat tire, which caused the wheel’s metal rim to scrape against the roadway. Authorities believe that sent sparks flying, USA Today reported in August.  

Dry debris nearby caught fire. Eventually, this blaze consumed an area three times the size of Washington, D.C., according to CalFire. That’s the state’s wildfire-fighting agency. The flames spread from forest to neighborhoods. And by the time it finally died out, the fire had claimed 7 lives and 1,604 homes and other structures.

But the truly remarkable part: This inferno grew so strong that it unleashed a massive tornado.

Wildfires can make for wild weather

Roughly half of all land burned in U.S. wildfires in 2017 was in California, Montana, Nevada, Texas and Alaska. That’s according to a November 2018 report by the Insurance Information Institute. And because of California’s large and dense population, wildfires in this state are among the most costly, both in terms of damage and lives lost.

Much of California is dry almost year-round. Large parts of it also get quite hot. Winter is usually the wettest season. That’s when big Pacific storms carry the Pineapple Express — a river of moisture that develops in the middle atmosphere. These storms target the California coast with a seeming firehose of moisture. Those rains fuel the growth of vegetation.

a photo of firefighters with the Carr Fire in the background
The Carr Fire outside Redding, Calif., burned for more than five weeks. Among the more notable features of this immense and deadly fire was its generation of a true tornado. Indeed, it was the biggest twister in California history.
Brenna Jones, USFS Pacific Southwest Region 5 (CC BY 2.0)/ Flickr

In the spring and summer, winds from the west pull in cool air from the Pacific Ocean. That gives San Francisco its famous fog. These winds also force moist air up the mountains. But when it sinks back down on the other side of the state’s mountains, that air dries out. This desert-like air can suck the moisture out of anything it touches. So any dead plant matter begins to dry up. By mid-summer, much of the ground throughout the state is littered with brittle sticks and leaves. This becomes a powder keg of fuel for a fire to gobble up. Lightning, unattended campfires, discarded cigarettes and sparks from vehicle tailpipes — all of those can ignite the dry forest debris.

Farther inland, winds swirl clockwise around a semi-permanent system of high pressure that parks itself near Reno, Nevada. This sends occasional spurts of wind and dry air westward through the Santa Ana Mountains and Sierra Nevada. These so-called Santa Ana winds can top 97 kilometers (60 miles) per hour. They dry the air out and can fan the flames of a wildfire.

If they get big enough, wildfires can create their own weather. The largest of them suck in so much air that the entering winds can flow at speeds of up to 130 kilometers (80 miles) per hour. These winds also supply fires with plenty of oxygen, which the blazes need to burn.

Once in a while, a wildfire will reach so high into the atmosphere that it causes rain. That happens when the warm, steamy updraft carries water vapor to a level where this gas condenses and falls out as liquid droplets.

Some wildfires even produce lightning. Soot, smoke, ash and tree-formed hydrocarbons can become electrically charged as they interact with ice crystals above 7,600 meters (some 25,000 feet). The ice takes on a positive charge. Raindrops become negatively charged. This charge-producing phenomenon has a really long name: triboelectrification (TRY-boh-ee-LEK-trih-fih-KAY-shun). When the electrical charges between the ice and rain grow large enough, a lightning bolt can pass between them.

The Carr Fire churned up some particularly wild weather — a true fire tornado. And one key factor behind that was the speed of the storm’s updraft.

The evolution of a fiery ‘tornado’

The National Weather Service, or NWS, releases weather balloons to collect a vertical profile of the temperatures, humidity, wind speeds and barometric pressure as they rise through the atmosphere. One of these daily soundings was taken with a balloon sent up before sunrise from Oakland, Calif., on July 26.

The balloon’s instruments detected a thin layer of warm air at about 1,000 meters (3,280 feet). Known as an inversion layer, it tends to hold air near to the ground from rising high into the atmosphere. In the Carr Fire, this “cap” trapped hot smoke close to the ground.

As energy continued to build beneath the inversion, the hot air pushed upward. That caused the cap to rise… and rise… and rise some more. This occurred all morning and afternoon. Around dinnertime, those hot gases had lifted the inversion layer to right around 6,100 meters (20,000 feet).A 3D weather model showing how the Carr fire firenado was formed

A 3D weather model showing how the Carr fire firenado was formed

A 3D weather model showing how the Carr fire firenado was formed

Then, around 7:20 p.m., the fire won. Two upwardly rising plumes of hot smoke and gas punctured through the cap. Within a half hour, these updrafts soared explosively — doubling in height to 12,800 meters (42,000 feet). That’s above the altitude at which jet airliners fly.

When the updrafts busted through the cap, they spanned multiple layers of the atmosphere. Wind shear shoved the budding storm clouds in many different directions. There also was plenty of rotational energy in the atmosphere — what's known as vorticity. In short order, he towering updrafts started to spin.

As the fires grew in height, the rotation of winds within them became more intense. As this rotating column of air was vertically stretched the conservation of angular momentum came into play. Think of an ice skater twirling. As she draws in her arms, she spins faster. The same thing happened here. The rapid doubling in height of the updrafts stretched the columns of spinning air. As their radius shrunk, they rotated faster. Before long, the fire clouds were spinning like a top.

It was the southern storm “cell” — one individual updraft — that produced the fiery tornado. At times this cell approached 0.8 kilometer (a half mile) wide. It became the first documented firenado in U.S. history.

A fire tornado is a true tornado. It’s born from rotating clouds and then reaches down from the clouds. Its winds are incredibly powerful, and it can have an impressive, potentially deadly impact. Plus, a firenado is incredibly rare.

News accounts might give you a different impression, though. They sometimes use the term firenado to describe something very different — a firewhirl. These are much, much smaller than a firenado.

Such small whirling masses of air usually are no more than a meter or two (up to 8 feet) across. Wildfires can spew these whirls of twirling, fiery debris by the dozen. One may even form over backyard campfires. They tend to have the same strength as leaf swirls on a gusty, fall day, and last less than a minute. More importantly, they’re not connected to a cloud. They just spin up from the ground in response to intense heat at the surface.

How strong was the Redding firenado?

After receiving reports of significant damage in the wake of the Redding fire’s tornado, the NWS Sacramento office sent out a team of meteorologists to investigate. One NWS tweet on August 2 noted that: “Preliminary reports include the collapse of high tension power lines, uprooted trees, and the complete removal of tree bark.” Its experts also had found evidence of winds in excess of 230 kilometers (143 miles) per hour.

The event met the American Meteorological Society’s definition of a tornado. AMS characterizes a tornado as a “rotating column of air, in contact with the surface, pendant from a cumuliform cloud.” The word cumuliform means a cloud with a powerful updraft. The July fire tornado was rooted in the massive cloud — one that were rotating. It also was fed by an intense updraft. And it was attached to a rapidly-growing fire-generated “cumuliform” cloud. It was, in fact, a cumulonimbus cloud.

Scientists use the Enhanced Fujita Scale to rank the strength — wind speed and destructive force — of tornadoes on a 0 to 5 scale. The Carr Fire’s tornado was a powerful EF-3. Most of the thousand or so U.S. tornadoes that touch down each year are EF-0’s or EF-1’s. Fewer than 6 in every 100 reach an EF-3 or higher.

California had seen two EF-3's in the 1970s. But neither was more than 60 meters (200 feet) wide. The Carr Fire tornado was 12 times that broad. Indeed, the Redding firenado was the strongest tornado of any type ever recorded in California.

The first recorded fire tornado was Down Under

On January 18, 2003, lightning sparked a wildfire near Canberra, Australia. Its smoke produced a cumulonimbus cloud. And like the system in Redding, the clouds grew into a supercell thunderstorm.

The Australian wildfire produced winds of up to 130 kilometers (80 miles) per hour. This challenged efforts to curb its growth. Jason Sharples is a fire scientist at the University of New South Wales in Sydney, Australia. He and three other scientists described this fire’s tornado in a 2013 paper. At some point, they note, clouds associated with the violent fire began to rotate. This spawned a terrifying twister. It was even worse than California’s. Though it stayed mainly over open countryside, it did level one neighborhood.

Jim Venn, a resident of the suburb of Wanniassa, captured the twister in a photograph from his back deck. The scientists then used math to analyze the photo to estimate the size of the cyclone’s rotating structure. They gauged the updraft speed of the tornado at an enormous 200 to 250 kilometers (124 to 155 miles) per hour. That’s enough to lift and toss a vehicle. It may come as no surprise, then, that this funnel was able to throw the 7-metric-ton (15,000 pound) roof of a water tower more than 0.8 kilometer (a half mile).

The tornado, which touched down six times, was also captured on video. The scientists argue it “meets the definition of a tornado.” It also appears to stand alone, with the Redding event, as the only two true tornadoes born of fire.

The 2003 Mount Arawang fire tornado in Australia. The emergence of its funnel occurred as the videographer was shooting the event. The firenado exhibited a strong upward motion within the whirling vortex.
The Weather Channel

And now come reports that another firenado may have sprung to life on November 9. It was at the edge of the deadly Woolsey Fire in Malibu, Calif. Something tore up trees and pulled the posts for power lines out of the ground. And video showed a clockwise-spinning vortex.

That rotation, however, is opposite to the spin direction of most tornadoes in the northern hemisphere. A later analysis of Doppler radar now suggests this furious funnel may have been a landspout — a twister-like vortex with a tornado’s strength. This blazing cyclone appeared to contain winds of 129 to 153 kilometers (80 to 95 miles) per hour.  It likely formed in response to small eddies (circling winds) moving downhill and gathering strength. Unlike most full-fledged tornadoes, this twister’s circulation was shallow. It gathered and lofted enough loose debris to be picked up on radar. Though scary, it wouldn’t have been a firenado.

This video showed the apparent landspout that developed as part of the November 2018 Woolsey Fire around Malibu, Calif. This trickster twister sported a clockwise-spinning vortex. That spin rotation is opposite to the direction of most tornadoes in the northern hemisphere.
Karen Foshay, KCET/ABC

Power Words

(for more about Power Words, click here)

air mass     A large volume of air, sometimes covering many hundreds or thousands of square kilometers (square miles), that typically has a consistent temperature or water-vapor content. Air masses are often classified by their source, such as continental, arctic or tropical. Air masses and other weather systems are steered across Earth’s surface by jet streams and by differences in atmospheric pressure.

angular momentum     A moving object is said to have momentum. When that object is rotating, the term becomes angular momentum. And that movement will not alter its speed unless acted on by another force.

ash     (in geology) Small, lightweight fragments of rock and glass spewed by volcanic eruptions.

atmosphere     The envelope of gases surrounding Earth or another planet.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

condense     To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

conservation of angular momentum     The angular momentum of a system — the energy used to power rotation — will not change unless acted on by some outside force. This holds even as movements within the system may change (for instance, as a rotating figure skater extends her arms and legs toward the body or out from it).

cumulonimbus     A cloud forming a towering mass with a flat base at fairly low altitude and often a flat top. These clouds can give rise to thunderstorms.

cyclone     A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.

debris     Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

develop     To emerge or come into being, either naturally or through human intervention, such as by manufacturing.

diameter     The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

eddy     A circular motion that develops in some liquid or gas and that moves in a direction opposite to the main current. This may create a whirlpool.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).

factor     Something that plays a role in a particular condition or event; a contributor.

fire tornado (or firenado)      A true tornado that can suddenly develop from conditions that accompany a wildfire. These cyclones stretch down from a rotating cloud base above. They are different from firewhirls — far smaller, twirling whirlwinds of fiery debris that often rise up from a blaze on the ground.

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

hydrocarbon     Any of a range of large molecules containing chemically bound carbon and hydrogen atoms. Crude oil, for example, is a naturally occurring mix of many hydrocarbons.

inversion layer     (in atmospheric science) A layer of Earth’s atmosphere in which the usual temperature pattern of air temperature falling with height becomes reversed. This prevents a mixing of warm and cold air as warm air attempts to rise. Such conditions may trap air — and pollutants — relatively close to the ground so that it can’t disperse and become diluted.

landspout     The land-based equivalent of a waterspout, it’s a cyclone — sometimes little more than a towering dust whirl — that is not associated with clouds overhead.

litter     (in biology) Decaying leaves and other plant matter on the surface of a forest floor.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

meteorologist     Someone who studies weather and climate events.

moisture     Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.

momentum     A measure of the motion of something, made by multiplying its mass and velocity. Changing the speed or direction of an object will also alter its momentum.

National Weather Service     An agency of the National Oceanic and Atmospheric Administration. Created in 1870, its current role is to collect weather, precipitation and climate data. It also issues forecasts and warnings 24 hours a day for the entire United States, focusing on signs of possible conditions that could threaten lives and structures.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

phenomenon     Something that is surprising or unusual.

plume      (in environmental sciences) The movement of some gas or liquid, under the direction of gravity, winds or currents. It may be in air, soil or water. It gets its name from the fact that it tends to be long and relatively thin, shaped like a large feather.

policy     A plan, stated guidelines or agreed-upon rules of action to apply in certain specific circumstances. For instance, a school could have a policy on when to permit snow days or how many excused absences it would allow a student in a given year.

preliminary     An early step or stage that precedes something more important.

radius     A straight line from the center to the circumference of a circle or sphere.

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

resident     Some member of a community of organisms that lives in a particular place. (Antonym: visitor)

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sink     (in biology) Some part of an ecosystem or the environment that serves as a storage depot for some chemical. For instance, trees or the soil can become a sink for the carbon released into the atmosphere.

smoke     Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.

soot     Also known as black carbon, it's the sometimes oily residues of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. Soot particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.

sounding     A measurement of various conditions of the atmosphere, usually with instruments carried aloft by a balloon; a measurement of the depth of the ocean or other body of water with a so-called sounding line; or a sampling of the opinions of people about a particular issue.

supercell     (in atmospheric science) A weather system with strong rotating clouds that is fueled by a long-lasting updraft of air. Such systems can generate hail — or even tornadoes.

tornado     A violently rotating column of air extending from the ground to a thunderstorm above.

tweet     Message consisting of 140 or fewer characters that is available to people with an online Twitter account.

updraft     A strong mass of air that rises quickly.

vegetation     Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.

vertical     A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.

volatile     Chemical that easily evaporates.

vortex     (plural: vortices) A swirling whirlpool of some liquid or gas. Tornadoes are vortices, and so are the tornado-like swirls inside a glass of tea that’s been stirred with a spoon. Smoke rings are donut-shaped vortices.

wake     An area of disturbed air or water left behind an object (such as a boat or animal) moving through it.

water vapor     Water in its gaseous state, capable of being suspended in the air.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

wind shear     A turning of the winds or a change in their speed with height.

Citation

Journal: R.H.D. McRae et al. An Australian pyro-tornadogenesis event. Natural Hazards. Volume 65, February 2013, p. 1801. doi: 10.1007/s11069-012-0443-7.